For a variety of reasons, in the fabrication of pipe and tubing by rolling up a tubular form from a flat strip or skelp and seam welding the abutting edges, it is impossible to maintain precise control of the finished diameter. Particularly in larger diameters and where lighter gauge material is used, for example in diameters above 150 millimetres or where the wall thickness is less than 2% of diameter, pipe and tubing fabricated in this way may not be perfectly round. Some variation from straightness is also frequently experienced. It is well known that standards for some forms of pipe and tubing prescribe quite liberal tolerances.
Many applications exist in which pipe and tubing must meet precise specifications in relation to diameter, roundness and straightness and a variety of methods has therefore been developed to correct defects in these criteria. Where the diameter of pipe or tubing has to be increased, it is common to pass a cylindrical die of some suitable hard material and having an external diameter somewhat greater than the internal diameter of the pipe or tube through the lumen of the pipe or tube to stretch it. Where more than a minor correction is required, consecutive passes of dies of increasing diameter may be required, the internal surfaces of the pipe or tube lumen may require lubrication, scoring of the internal surfaces is common and some degree of wall thinning will occur. The process has the advantage of being operable on a continuous basis. In another method, the internal diameter of pipe or tubing is increased by subjecting the interior of short lengths to hydraulic pressure to expand it into an enclosing female die. Use of this method is normally confined to short lengths of pipe or tubing and has the disadvantages of slowness and the fact that it cannot be operated on a continuous basis. Both methods are well known in the art.
Where the diameter of pipe or tubing is required to be decreased, it is common to roll it down by passing the pipe or tubing through a plurality of concave rollers arranged such that their diameters extended meet at a common point and with their collective concavities more or less forming a complete circle slightly smaller than the final diameter of pipe or tubing required. Equally-spaced rollers are supported on shafts parallel to tangents to the surface of the pipe and tubing and are driven in rotation while the pipe or tubing to be resized is fed between them and is thereby cold worked to a smaller diameter. Unless the pipe or tubing is stretched at the time, some degree of wall thickening will occur.
One example of this method appears intended for use with only pipe or tubing of smaller diameters and the fact that the method includes a provision for final sizing to be performed by drawing the rolled pipe or tube through a female sizing die is indicative of the limited control of worked diameter available. During this method only relatively small decreases in diameter may be achieved in a single pass, normally of the order of 0.2 to 0.4 mm, that what is effectively a wiping action of the sides of the roller concavities may scuff or mar the external surfaces of pipe or tubing (an important factor in stainless steel products), and the fact that the method is relatively ineffective in large, relatively thin-walled pipe or tubing. The scuffing or marring of external surfaces is particularly pronounced in larger diameter pipe or tubing where the method is normally performed using only two rollers having deep concavities. Obviously, as suggested in the example cited, the diameter of pipe or tubing may be reduced by drawing it through a female sizing die. Where this method is employed, the pipe or tube may require lubrication, the external surface of the pipe or tubing is frequently scored by asperities in the die or picked up by the die and some wall thickening and elongation may occur. In another method, both internal and external dies are used in what is normally a second or third manufacturing operation.
Another example of diametral reduction by rolling, a short length of pipe or tubing is rotationally supported by clamps only at the ends and a plurality of cylindrical rollers is brought to bear against the outer surface of the length of pipe or tubing while it is rotated, thereby reducing its diameter and, if required, rendering it into tapered form. The method is applicable only to short lengths of pipe or tubing and obviously cannot be operated as a continuous process. Of relevance is a method in which thin-walled metallic tubing is formed from a solid blank in an Assel rolling mill. In this case, provision is made to vary the wall thickness of the formed tubing by adjusting the radial positions of a plurality of forming rollers. Adjustment is effected by increasing the skew of short shafts upon which the forming rollers are rotationally supported, thereby radially displacing the rollers inwardly or outwardly. The ends of the short shafts are rotationally supported in suitable bearings accommodated within the ball parts of ball and socket joints, which ball parts move in complementary sockets to permit skewing of the shafts. The rollers are short and are provided with shoulders which work on the blank from which the tubing is formed.
In many tube rolling methods, a mandrel is inserted into the lumen of a tube to be rolled and the tube worked by a plurality of rollers against the mandrel. Applications are also common in which laminated pipe or tubing is made by drawing one piece of pipe or tubing into the lumen of another. Where, for example, the inner pipe or tube is made from a polymer material, it is common to temporarily reduce its diameter by passing it between concave rollers or through a female sizing die in the manner described and, when positioned inside a pipe or tube of larger diameter, expanding it by the application of internal fluid pressure to make a tight fit within the outer pipe or tube. Additionally, to ensure a more secure capture of the inner pipe or tube, the outer pipe or tube may subsequently be reduced in diameter using one of the methods described. Where both the inner and outer pipes or tubes are of metal, the inner is captured simply by reducing the diameter of the outer.